HI Broadband is not where you run into the trouble on any of these circuits. It’s always what happens within a decade or two past cutoff or inside the pass band.
Bob > On Aug 1, 2016, at 4:50 PM, Scott Stobbe <[email protected]> wrote: > > The broadband thermal noise at a circuit point with a cap is always kT/c > > On Monday, 1 August 2016, Bob Camp <[email protected]> wrote: > >> Hi >> >> If you wire up all the possible circuits and check them all out … the >> answer is that big C / small R wins. Big R gets you into resistor noise >> issues >> and stray pickup. >> >> Bob >> >>> On Aug 1, 2016, at 4:16 PM, David <[email protected] <javascript:;>> >> wrote: >>> >>> This duplicates the problems encountered when trying to quantify low >>> frequency noise from a voltage reference; it is difficult to make an >>> low frequency high pass filter with lower noise than the lowest noise >>> references and the capacitor is the problem. >>> >>> In Linear Technology Application Note 124, Jim Williams discusses the >>> problems with electrolytic capacitors for this type of application. I >>> have read that you *can* get away with aluminum electrolytics if you >>> grade them for low leakage and low noise. The dielectric absorption >>> is also a problem unless you can wait hours for best performance. >>> >>> What about the alternative of buffering the signal with a low noise >>> low input bias current operational amplifier so that a large film >>> capacitor can be used instead? Is the low frequency noise of a good >>> operational amplifier still too much? What about a chopper stabilized >>> amplifier without suitable output filter? >>> >>> On Mon, 1 Aug 2016 11:46:51 -0400, you wrote: >>> >>>> Hi >>>> >>>> >>> .. until you discover that you picked the *wrong* capacitor manufacturer >> and you have >>>> more noise from leakage in the cap than you did to start out with :) >> In general “big C and >>>> small R” is the better solution than “big R and small C”. >>>> >>>> The pesky part is that with electrolytic caps, the whole “noise >> current” thing changes as >>>> the voltage moves around. You go to measure things and by the time the >> gear is set up, >>>> the noise has dropped. Turn it all off, come back the next day and it’s >> noisy again. >>>> >>>> An even more subtle issue can be capacitor temperature coefficient on >> really long Tau filters. If C >>>> changes (due to temperature fluxuations) faster than the settling time >> of the filter, you get noise. Charge >>>> is the same so delta C gives delta V. >>>> >>>> I *wish* I could tell you that was all purely theoretical. >> Unfortunately it’s based on empirical data >>>> collected in the “how could I be so stupid” fashion. >>>> >>>> Bob >>>> >>>>> On Aug 1, 2016, at 11:21 AM, KA2WEU--- via time-nuts < >> [email protected] <javascript:;>> wrote: >>>>> >>>>> A good filter in the cable is highly recommended, 5 KOhm & 1000 uF >> cleans >>>>> many things >>> _______________________________________________ >>> time-nuts mailing list -- [email protected] <javascript:;> >>> To unsubscribe, go to >> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >>> and follow the instructions there. >> >> _______________________________________________ >> time-nuts mailing list -- [email protected] <javascript:;> >> To unsubscribe, go to >> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >> and follow the instructions there. >> > _______________________________________________ > time-nuts mailing list -- [email protected] > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
